1. Field of the Invention
The present invention relates to improvements in a rotating cooling wheel such as a brake drum or disk rotor for use in braking devices such as drum brakes or disk brakes, a method of manufacturing the same, a drum brake and, more particularly, to improvements in vibration characteristics of the rotating cooling wheel to reduce so-called brake noises.
2. Description of the Related Art
The drum brake is one of the conventional brake devices. The drum brake is applied by making use of friction force caused when linings which are stuck on substantially circular brake shoes supported to a member on the car body side are forced to contact to an inner peripheral surface of a substantially cylindrical brake drum which acts as a rotating cooling wheel rotated together with a wheel.
More particularly, in the drum brake, anchors are secured to a back plate fixed on the car body side to be positioned in the inside of the inner peripheral surface of the brake drum and receive braking torque. The opposing ends of a pair of brake shoes are fitted swingably to the anchors. Respective pieces of a pair of brake shoes are arranged along the inner peripheral surface of the brake drum. When the brake is not applied, the linings stuck on the outer surfaces of the brake shoes are opposed to the inner peripheral surface at a predetermined distance.
A wheel cylinder and a return spring are provided to the other ends of a pair of brake shoes which are opposed to each other on the side opposite to the anchors side. When hydraulic pressure is supplied to the wheel cylinder, a pair of pistons which are provided to move back and forth in the lateral direction are displaced to extend. Accordingly, the other ends of a pair of brake shoes are opened laterally to be pushed to the inner peripheral surface, so that the linings are pushed to contact the inner peripheral surface 1a. A rotating force of the brake drum is converted into frictional heat in their contacting areas to thus apply the brake.
In the drum brake of this type, frictional vibration caused on frictional surfaces when frictional heat is generated by forcing the linings to contact to the inner peripheral surface of the brake drum would vibrate the brake drum to thus excite natural vibration of the brake drum. As a result, unpleasant brake noises would be generated in some cases.
The cylindrical body, circular disk, or the like having the same thickness, like the ordinary brake drum, has its eigenmode to exhibit a vibration mode in which the outer peripheral portion bending-vibrates to generate, for example, diametral two-node mode (see FIG. 1A), diametral three-node mode (see FIG. 1B), diametral four-node mode (see FIG. 1C), diametral five-node mode (see FIG. 1D), diametral six-node mode (see FIG. 1E), or the like. Multiple roots are always included in the eigenmode since the brake drum is symmetrically formed with a center axis of rotation (i.e., an axis extending in the direction perpendicular to a face of the drawing sheet to pass through a center of the brake drum in FIG. 6) as a center. The expression "multiple roots are included" means that other eigenmode having the same profile which is shifted in phase by a half period in the circumferential direction is present at the same frequency in addition to one eigenmode respectively shown in FIGS. 1A to 1E.
More particularly, if the peripheral surface of the brake drum is vibrated in its stationary state, it seems that a plurality of eigenmodes are present on the brake drum since, even if any position is selected as an excitation point in the circumferential direction, such a response mode appears that the excitation point always serves as an antinode because of symmetry of the brake drum with respect to the center axis of rotation. However, if axes shown by dot-dash lines in FIGS. 1A to 1E are assumed along the diameter, it may be considered that these axes rotate around the brake drum. In this event, it can be deduced from an aspect of vibration that two eigenmodes, i.e., the eigenmode shown in FIGS. 1A to 1E and the eigenmode having the same profile shifted by a half period exist on the brake drum. Such cases may be considered as "multiple roots are included".
On the contrary, if the peripheral surface of the brake drum is vibrated in its rotating state as in a case where the actual drum brake is applied, a response mode in which the excitation point serves as an antinode may be observed as if it stands still in a space irrespective of the brake drum (such response mode is called as a "spatially fixed mode"). Since such spatially fixed mode means that the antinode of the eigenmode always serves as the excitation point, the mode can be excited with highest efficiency to maximize sound radiant efficiency. For this reason, such spatially fixed modes may often constitute a major factor for actual brake noises.
Such a phenomenon is not limited to the drum brake, but is true similarly of the disk brake wherein a disk rotor serving as the rotating cooling wheel rotated together with the wheel is held by brake pads since the disk rotator is formed of the circular disk having symmetry with respect to the center axis of rotation.
As the prior art pointing out such phenomenon, there has been disclosed Patent Application Publication (KOKAI) 56-52633 (first prior art) which has also been proposed previously by the inventors of the present invention. In other words, such first prior art is characterized in that at least three thick portions are formed on the outer peripheral surface of the brake drum at a constant distance in the circumferential direction. For instance, in the case that three thick portions are formed, these thick portions are located at antinodes of one of two eigenmodes in the diametral three-node mode, as shown in FIG. 2A, and act as additional masses to thus decrease the resonance frequency, but these thick portions locate at nodes of the other of two eigenmodes, as shown in FIG. 2B, and act as rigidity reinforcing members to thus increase the resonance frequency. Then, multiple roots are separated so that the spatial fixed mode can be eliminated to reduce sound radiant efficiency. As a result, brake noises can be suppressed. The concept to separate multiple roots by adding the mass has been introduced in a literature, "Paper No.88-0622A in Transactions of the Japanese Society of Mechanical Engineers(Section C), Vol.55, No.512 (1984-4)".
On the contrary, as other prior art pointing out the above phenomenon, there has been disclosed Utility Model Application Publication (KOKAI) 62-89537 (second prior art). In other words, the second prior art concerning the drum brake is characterized in that elastic members are intervened between the bottom of the brake drum and the wheel disk, the road wheel being constituted by the wheel disk as well as rims, at an equal distance on a concentric circle and that the elastic members are clamped by pressing the wheel disk to the brake drum. For instance, in the case that six elastic members are intervened, these elastic members locate at antinodes of one of two eigenmodes in the diametral three-node mode, as shown in FIG. 4A, and act as elastic members against vibration of the brake drum to thus increase the resonance frequency, but these elastic members locate at nodes of the other of two eigenmodes, as shown in FIG. 4B, and do not act as elastic members not to vary the resonance frequency. Then, multiple roots are separated so that the spatial fixed mode can be eliminated to reduce sound radiant efficiency, and consequently brake noises can be suppressed.
If three thick portions are provided at a constant distance, for example, in the structure in the first prior art, it is surely feasible to suppress brake noises in some degree since multiple roots included in the diametral three-node mode can be separated. However, since the order of the diametral node mode in which multiple roots are separable is determined in compliance with the number of the formed thick portions, sometimes unpleasant brake noises are still generated because of remaining diametral node modes in which multiple roots are not separable.
FIG. 3 is a graph illustrating the results derived from calculation according to the finite element method (FEM) every order of the diametral node mode to show differences in eigenvalues between the brake drum in which six thick portions are formed on the outer surface in accordance with the first prior art and the ordinary brake drum. As can be appreciated from the results, multiple roots can be separated by six additional masses in the mode of multiples of three such as diametral three-node mode or diametral six-node mode, but they cannot be separated as for other order even if natural frequency can be reduced.
Conversely, if the structure according to the above second prior art is adopted, the brake noises can be suppressed in some degree since multiple roots in the diametral three-node mode can be separated by interposing six elastic members between the brake drum and the wheel disk at a constant distance, for example. However, since the order of the diametral node mode in which multiple roots are separable is determined in compliance with the number of the intervened elastic members, unpleasant brake noises may still be generated because of remaining diametral node modes in which multiple roots are not separable.
FIG. 5 is a graph illustrating the results derived from calculation according to the finite element method (FEM) to show how eigenvalues of the diametral node mode can be separated in the drum brake in which six elastic members are intervened between the brake drum and the wheel disk in accordance with the second prior art. As apparent from the results, although multiple roots can be separated by six elastic members in the mode of multiple of three such as diametral three-node mode or diametral six-node mode, they cannot be separated as for other orders even if natural frequency can be reduced.
In other words, once the number of additional masses or elastic members is determined according to either the first prior art or second prior art, a separable order of multiple roots is decided. Although set forth in the above literature, relationships between the number N of the additional masses or elastic members and order i of the efficient diametral node mode are given, as shown in the following Table I. In Table I, a .largecircle. mark represents a separable multiple root. According to Table I, in the case of two additional masses or elastic members, improvement can be obtained throughout all orders. In practice, not only can little effect be merely obtained in higher modes, but also the frequency cannot be tuned freely in higher modes if two additional masses or elastic members are employed.
TABLE I ______________________________________ N 2 3 4 5 6 7 8 9 10 ______________________________________ 2 .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. 3 .largecircle. .largecircle. .largecircle. 4 .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. 5 .largecircle. .largecircle. 6 .largecircle. .largecircle. .largecircle. 7 .largecircle. 8 .largecircle. .largecircle. ______________________________________
In the actual drum brake, because of differences in operating conditions such as operating hydraulic pressure for the wheel cylinder and lining changes with elapsed time, the frequency component of friction vibration caused between the inner peripheral surface of the brake drum and the linings is varied. Therefore, reduction in the vibration as with particular order of the diametral node mode, like the prior arts set forth in the above official gazettes and the literature, cannot successfully cope with sufficient reduction in the brake noises in any circumstances.